Ballistocardiographic measurements conducted with accelerometers or angular rate sensors measure the movements of a human body which arise from the pumping action of the heart. Two mechanical effects associated with heartbeats can typically be detected: a recoil effect on the chest caused by the pumping motion itself, and a recoil effect on the whole body caused by the ejection of blood into arteries across the body. Both of these recoil effects will be called “heartbeat recoils” in this disclosure.
Most movements which human beings generate on their own volition are stronger and more extensive than a heartbeat recoil. Ballistocardiographic measurements therefore typically require that the person on whom the measurement is conducted should be at rest, preferably lying down. Good measurement data can often be obtained when the person is sleeping.
Recoil effects can be detected either directly from the body or indirectly from a surface which is in physical contact with the body and moves with it. This surface may, for example, be a mattress where the person is resting. When the test subject is a fetus, the surface may be the mother's belly. In this disclosure, the term “recoil surface” will be used as a general term for a surface where a ballistocardiographic measurement can be performed.
Even when the person on whom a ballistocardiographic measurement is conducted is lying still, heartbeat recoils are very small movements and the sensors used in these measurements must be very sensitive. This problem has been resolved with modern MEMS acceleration sensors and gyroscopes, which have sufficiently high resolution for ballistocardiographic measurements.
The interpretation of ballistocardiographic measurements is aided by the periodicity of the heartbeat. The person on whom the measurement is performed may move around as she rests or sleeps, but such movements are not periodic. The periodic heartbeat recoil signal, which usually exhibits a frequency of 50-200/minute, can be distinguished from non-periodic movements and from periodic movements with frequencies outside of this range, such as respiratory movement. However, if two separate heartbeat recoil components are present in the same ballistocardiographic signal, they can be hard to distinguish from each other.
The heartbeats of newborn infants and unborn fetuses are often monitored for medical reasons. A drop in their heart rate indicates a decrease in oxygen saturation in the blood stream. If an infant's heart rate drops below 60 beats per minute, he or she may be at critical risk of cot death. A drop in a fetus' heart rate is an indication of similar risks. Immediate action is then required to avoid a fatal outcome.
Continuous automated monitoring of infant and fetus heartbeats, especially during sleep, is a reliable preventive measure against such risks. Automated monitoring systems can comprise an alarm function which alerts a nearby adult of a sudden drop in an infant's or a fetus' heart rate.
Fetus heart rates can be monitored in hospitals with ultrasonography and infant heart rates with sensors placed on the infant's body. However, neither of these methods can be easily implemented for continuous heart rate monitoring at home. Ultrasonography requires complex equipment and a human operator, while wearable sensors can be uncomfortable to an infant.
In contrast, automated ballistocardiographic monitoring of infant and fetus heartbeats can be performed continuously without inconvenience to the infant or fetus. A ballistocardiographic sensor can, for example, be attached to the bed or mattress where an infant sleeps, or to the belly of a pregnant mother.
Ballistocardiographic measurements can be performed on a lone infant sleeping in a bed with methods known from the prior art. Document US20080077020 A1 discloses a method and apparatus for monitoring the vital signs of an infant sleeping in a crib. The method utilizes a ballistograph, which in this case is a fluid-filled pad or mechanical plate equipped with pressure sensors or other sensors which record the heart rate.
However, infants often sleep in the same bed as one or more adults. This produces a measurement problem: the weak heartbeat recoils caused by an infant's heart cannot be read directly from the ballistocardiographic signal because they are mixed with adult heartbeat recoils. The same measurement problem is present when fetus heartbeats are measured with a ballistocardiographic sensor. The mother's heartbeat recoil is significantly stronger than the fetus' heartbeat recoil.
In the presence of two heartbeat recoils, there will be two overlapping periodic components in the ballistocardiographic signal. It is not possible to distinguish the much weaker signal caused by an infant's heartbeat recoil from the stronger signal caused by an adult with prior art methods, such as the one disclosed in document US20080077020 for monitoring a lone infant.
Document US20100305481 A1 discloses a system for automatically monitoring the movement of a fetus. The method is based on detecting the movements and the heartrate of the mother through ballistocardiographic measurements, and using this information to distinguish the fetus' movements from the mother's movements. However, this document does not disclose a measurement of the fetus' heart rate.
Document WO2015036925 A1 discloses methods for determining the heart rate and respiration rate of a person from ballistocardiographic data measured from an acceleration sensor. But in situations where two heartbeats are present, one stronger than the other, only the stronger heartbeat can be identified with this method from the measurement signal.